This invention relates to high temperature secondary electrochemical cells and electrodes for such cells. In particular, it relates to electrochemical cells employing molten alkali metal halides as electrolyte, lithium or lithium alloy as negative electrode material and transition metal chalcogenides as positive electrode material. Representative electrochemical cells of this type are disclosed in U.S. Pat. Nos. 3,907,589 to Gay et al., 3,992,222 to Walsh et al., 4,386,019 to Kaun et al., 4,446,212 and 4,540,642 to Kaun and in ANL-84-93, "Li-Alloy/FeS Cell Design and Analysis Report".
Secondary electrochemical cells of this type can be used over long periods of times with numerous charging and discharging cycles. They have high capacity for energy storage and can be assembled into batteries of cells for a multiplicity of applications. Typical uses include principal and auxillary power sources for electric or other vehicles, storage of off peak power in conjunction with electric utilities, and power sources for remote locations in space or remote terrestial locations.
Considerable development work has been carried out in electrochemical cells of this type. The electrodes have been assembled in both the charged and discharged state and reaction products have been incorporated into the original electrode compositions as is described in U.S. Pat. No. 3,907,589 cited above. Various mixtures of electrode reactive materials have been investigated. For instance, additives in minor amounts of metal sulfides have been incorporated into positive electrodes with iron disulfide reactants as is shown in U.S. Pat. No. 3,992,222. However, none of these earlier efforts have produced an electrochemical cell with positive electrodes of consistently low impedance throughout its full discharge range. As an example, FIG. 3A of U.S. Pat. No. 3,992,222 shows relatively poor performance for an FeS.sub.2 - NiS positive electrode.
In applications requiring high power output over a short period of time, rapid discharge of the electrochemical cells must occur. In these applications, it is of importance to have constant impedance over the full discharge cycle of the selected electrochemical cell. Unfortunately, electrochemical cells with the transition metal chalcogenides as electrode material have not demonstrated sufficient consistency in impedance for good power source control.
One other drawback of electrodes with metal sulfide active material is the requirement of overcharge protection. Complex control systems such as shown and described in U.S. Pat. No. 4,079,303 to Cox and 4,238,721 to DeLuca et al. have been proposed to prevent damage to structural components in the cell during charging. Without such a system a battery could not be charged to full capacity.